Method for the generation and use of ferric ions

ABSTRACT

A process for generating a ferric ion solution. The process includes the steps of establishing in a vessel a population of acidophilic, chemoautotrophic bacteria which oxidize ferrous ions into ferric ions; supplying the bacteria with a feed solution of ferrous ions; maintaining the pH of the contents of the vessel below 3.0; and withdrawing the generated ferric ion solution from the vessel. The generated ferric ion solution can be used to oxidize metal or ore and to solubilize metal from ore, alloys or scrap. The ferrous ions produced during the oxidation or solubilization are recirculated into the feed solution to continue the process of generating ferric ions.

FIELD OF THE INVENTION

The invention relates to an apparatus and method for the generation anduse of ferric ions produced by acidophilic, chemoautotrophic bacteria.

BACKGROUND OF THE INVENTION

Naturally occurring bacteria are known to catalyze the dissolution ofminerals. It is widely accepted that certain chemoautotrophic bacteria,such as Thiobacillus ferrooxidans among others, catalyze the oxidationof ferrous ion (Fe⁺²) to the ferric (Fe⁺³) state and sulfide (S⁼ or S⁻)to sulfate (SO₄ ⁼), and utilize these low energy reactions for producingmetabolic energy for growth. Fe⁺³ is a chemically reactive ion whichoxidizes many metal or sulphur bearing materials. In the process, theferric ions are reduced to the Fe⁺² state and the sulfate reacts withwater to lower the pH. In essence, these types of bacteria are able togenerate a highly oxidative, acidic solution which is conducive to thedissolution of numerous materials such as those containing copper, iron,zinc, lead and mercury.

The leaching of materials such as ore using such chemoautotrophicorganisms, a process termed bioleaching, has attracted much attentionand interest in recent years due to increasing environmental awarenessand decreasing ore grades. In some situations, it is no longereconomically feasible to smelt low grade ores. Bioleaching is a morecost effective means of recovering metal from lower grade ores in thatbioleaching is less energy consumptive and presents a lowerenvironmental risk.

Some controversy exists as to how chemoautotrophic bacteria accomplishbioleaching. Current mining industry belief is that direct attachment ofthe bacteria to the ore is critical in leaching the ore. The bacterialadhesion is thought to be the initial step for oxidation. Most currentbioleaching techniques involve the acidification of an inoculum ofchemoautotrophic bacteria introduced to a pile of ore. Sulfuric acid andnutrients are then added to the ore to encourage the organism to oxidizethe minerals below.

The problem with such current approaches to bioleaching is that toxinsnative to the ore will be solubilized during the oxidation process. Suchtoxins include arsenic, mercury, cadmium and other metals andmetalloids. These metals and metalloids at low concentrations willdestroy the bacteria resulting in significant down-time waiting toacclimate new bacteria to the ore pile. Further, the chemoautotrophicbacteria utilized in bioleaching are typically mesophilic and grow attemperatures between 25° C. and 40° C. Bioleaching reactions areexothermic in nature and, as a result, much heat is released so that thecenter of the ore pile may reach temperatures as high as 90° C.Accordingly, the bacterial oxidation activity can only occur in thecooler, top layer of the ore pile.

Pile bioleaching as well as other processes involving bioleaching, oftenpresent harsh conditions for optimal bacterial activity. Bioleaching haslong been treated as a single unit process making optimization of theprocess a difficult task. Changes in the conditions within the pilefound to be advantageous in conventional chemical leaching may adverselyaffect the activity of the organism in bioleaching.

Since their discovery, chemoautotrophic organisms useful in bioleachingwere viewed by the mining industry as a promising and inexpensive meansof oxidizing various components of crude ore. Although several largescale attempts have been made to enhance the growth of these organisms,little commercial success has been realized. Invariably as growth of thebacteria proceeds, the dissolution of toxins kill the organisms. Inlarge scale operations, this leads to lost productivity as new bacteriamust be reintroduced and established in the leach pile. In addition,bacterial activity drops considerably during the cold weather months.While bioleaching with chemoautotrophic bacteria has been known for manyyears, commercial applications have yet to be adopted as a viableindustrial process due to the above problems.

Leaching can also be accomplished by allowing contact of the ferric ionsolution, produced chemically, with the material to be treated. Thisprocess, however, has not proven commercially feasible due to theprohibitive cost of purchasing or chemically producing the ferric ionsolution.

SUMMARY OF THE INVENTION

The invention described herein provides an apparatus and method for theproduction of ferric ions using chemoautotrophic bacteria. Thechemoautotrophic bacteria are continuously maintained in a bioreactorthrough the control of system parameters such as pH and cell density sothat a continuous output of ferric ions can be maintained indefinitely.

The produced ferric ions can be used in various applications such asleaching. For example, the ferric ions can be utilized in thedissolution of metal-bearing minerals for the purposes of metalrecovery, in the pretreatment of precious metal-bearing ion to removerefractory minerals, or in a cleaning process for coal. In effect, theinvention can be viewed as either a more cost efficient method by whichto produce ferric ions for leaching applications as compared to chemicalproduction of ferric ions or the invention can be viewed as separatingthe chemoautotrophic bacteria from the material to be treated whenperforming bioleaching.

With regard to the former, the invention provides for low costproduction of ferric ions that can be used in traditional leachingapplications. Through the use of chemoautotrophic bacteria that can becontinuously maintained in a bioreactor to produce ferric ions, the costof production of the ferric ions makes its use in commercial industrialapplication feasible.

With regard to the later view of the invention, and contrary to the viewof the mining industry, it is the belief that the chemoautotrophicbacteria do not need to adhere to the material to be treated in orderfor the oxidation of that material to occur. Accordingly, by treatingbioleaching as two operations, bacterial generation of ferric ions thenleaching, each process can be optimized separately in order to improverecovery and economics. The organisms are used to oxidize a solution offerrous iron in a controlled environment and then the resulting ferricions are useable in applications such as leaching. The invention therebyallows for the elimination of the destruction of a majority of thebacterial population during the leaching application. By separating themajority of the bacterial population from the material to be treated,the bacterial population is not destroyed by contact with toxic metalores. Any bacteria that may exit the bioreactor with the ferric ions maybe destroyed due to contact with the material to be treated.

With either view of the invention, and in most applications of theferric ion solution, the interaction of the ferric ions with thematerial to be treated results in the regeneration of ferrous ions thatcan be recycled to the bioreactor to serve as a continual source ofenergy for the growth of the chemoautotrophic bacteria. Any bacteriawhich exits the bioreactor and survives the subsequent application willalso be recycled to the bioreactor.

It is an object of the present invention to provide a method for thegeneration of ferric ions.

It is another object of the present invention to provide an apparatusfor the generation of ferric ions.

It is an another object of the present invention to provide a method forthe generation of ferric ions using bacteria.

It is an another object of the present invention to provide a processfor the continuous maintenance of a bacterial population whichcontinuously produces ferric ions.

It is another object of the present invention to optimize the generationof ferric ions from chemoautotrophic bacteria.

It is another object of the present invention to maintain the health andviability of a bacterial culture through monitoring of parameters suchas pH and cell density.

It is another object of the present invention to generate a ferric ionsolution at a low cost.

It is another object of the present invention to generate a ferric ionsolution for use in applications such as leaching.

It is another object of the present invention to provide a process forleaching of metal or sulfur bearing materials using ferric ions.

It is another object of the present invention to provide a bioleachingprocess that separates the bacteria from the material to be treated.

It is another object of the present invention to recycle ferrous ionsthat were generated during the use of ferric ions back to a bioreactorto be used as a feed solution for a bacterial population to produce moreferric ions.

It is another object of the present invention to provide a process togradually introduce a recycled ferrous solution to bacteria to acclimatethe bacteria to higher levels of chemicals known to be toxic to thebacteria.

Other features and advantages of the invention will become apparent tothose of ordinary skill in the art upon review of the following detaileddescription, claims, and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the continuous ferric ion generation systemembodying the invention;

FIG. 2 is an elevational view of a bioreactor;

FIG. 3 is a sectional view of the bioreactor; and

FIG. 4 is a schematic depicting the use of the bioreactor embodying theinvention.

Before one embodiment of the invention is explained in detail, it is tobe understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the drawings. Theinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Also, it is to be understood that thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, an apparatus and method for thecontinuous generation of ferric ions and its use and for the continuousmaintenance of a bacterial population to produce the ferric ions isshown in FIG. 1. The system includes a bioreactor 10 which is utilizedto produce ferric ions. The bioreactor 10 employs bacteria to generatethe ferric ion solution from a feed solution containing ferrous ions.

As shown in FIGS. 1-3, the bioreactor 10 includes a vessel or tank 12.In one embodiment, the tank 12 has a flat bottom 14, an annular sidewall 16 and a flat removable top 18 which is preferably removed duringoperation of the bioreactor 10. It should be noted that otherconfigurations of the tank 12 can be used in practicing the invention.

The tank 12 should be constructed of materials which are resistant tocorrosion such as by ferric sulfate and sulfuric acid. Further, the tankshould not contain any materials which may be toxic to the bacterialpopulation. Preferably, the bioreactor 10 is constructed of fiberglassreinforced plastic and is a premium grade Derakane® 411-45 vinyl esterresin in a corrosion barrier, an isophthalic polyester resin in astructural layer and a single C-glass veil in the corrosion barrier. Thebioreactor 10 can be of varying volumetric capacities such as, forexample, 1 liter, 40 liters, 4000 liters, or 40,000 liters dependingupon the volume of the ferric ion solution required for a particularapplication.

An example of the tank 12 is model number C-CFV-4-590 manufactured byBelding Tank Technologies, Inc. of Belding, Michigan which has a volumecapacity of 2400 liters. The flat top 18 is preferably 0.250" thick witha 0.010" single C-glass veil in an inner layer, a 0.100" corrosionbarrier, and a 0.140" structural layer. The side wall 16 is preferably0.250" thick at the top portion of the side wall 16 with a 0.010" singleC-glass veil in an inner layer, a 0.100" corrosion barrier, and a 0.140"structural layer. The bottom portion of the side wall 16 is preferably0.500" thick. The flat top 18 of the tank 12 is preferably 0.250" thickwith a 0.010" single C-glass veil in an inner layer, a 0.100" corrosionbarrier, and a 0.140" structural layer.

The tank 12 defines an interior chamber 20 for housing the bacterialpopulation. Preferably, the bacteria used to generate the ferric ionsare of the genus Thiobacillus, and more preferably are of theThiobacillus ferrooxidans species such as strain ATCC 19859 depositedwith the American Type Culture Collection. The invention will hereafterbe described in operation with the bacteria of the species Thiobacillusferrooxidans. However, it should be noted that bacteria that areacidophilic, chemoautotrophic and utilize ferrous iron as an oxidizableenergy source can be utilized with this invention. Examples of otherorganisms, which are not intended to be limiting, that meet the abovecriteria include Leptosprillum ferrooxidans, organisms of the genusSulfolobus such as Sulfolobus acidocaldarius, and Sulfobacillusthermsulfidooxidans. Thiobacillus ferrooxidans is a chemoautotrophic,obligate, mesophilic aerobe which is approximately 1 μm in length,rod-shaped and mobile. Thiobacillus ferrooxidans derive energy from theoxidation of ferrous iron (Fe²⁺) in the presence of oxygen at pH valuesbetween 1.5 and 5.0, with the optimum pH reported to be below 2.2.Thiobacillus ferrooxidans utilizes carbon dioxide as its sole source ofcarbon for growth. As a result of the oxidation of ferrous iron at lowpH values, a highly oxidizing solution containing ferric ions isproduced which is useable as a leachant for metal or sulfur bearingmaterials.

A population of Thiobacillus ferrooxidans can be prepared as follows.Thiobacillus ferrooxidans is grown and maintained as described inLaCombe-Barron and Lueking, Growth and Maintenance of Thiobacillusferrooxidans Cells, Appl. and Environ. Microbiol., 56:2801-2806 (1990),which is incorporated by reference herein. The organism is grownchemoautotrophically on ferrous sulfate based minimal medium containing12 grams of ferrous iron per liter. Sterile media is prepared employingtwo solutions, a ferrous sulfate solution and a basal salts solution.The ferrous sulfate solution cannot be sterilized by autoclaving becausethe ferrous iron present degrades under high temperature and pressure.Therefore, the two solutions are prepared separately and combinedfollowing sterilization.

The ferrous sulfate solution is prepared by dissolving 59.7 grams offerrous sulfate heptahydrate in 520ml of distilled water. 10N sulfuricacid is added to the solution in order to obtain a pH of 1.90. Thesolution is filter sterilized using a 0.45 μm nitrocellulose or acetatefilter and a Millipore® filter apparatus.

The basal salts solution is prepared by dissolving 3.5 grams ammoniumsulfate, 0.058 grams potassium phosphate dibasic, 0.116 grams potassiumchloride, 0.058 grams magnesium sulfate heptahydrate and 0.00168 gramscalcium nitrate in 420ml of distilled water. The pH of the solution isadjusted to 1.90 by the addition of 10N sulfuric acid.

Thiobacillus ferrooxidans is grown in 18×150mm culture tubes or in 500mlserum bottles containing 10ml and 300ml volumes of ferrous sulfateminimal medium respectfully. A population of the Thiobacillusferrooxidans is then transferred to and grown in an inoculum tank 22,which is smaller in volume than the bioreactor 10, before beingtransferred to the bioreactor 10. Preferably, a series of inoculum tanks22 are employed using a 10% pre-inoculum level before the bacteria aretransferred to the bioreactor 10. For example, if a 4000 literbioreactor is employed, the bacteria would be maintained in a 40 literinoculum tank, then transferred to a 400 liter inoculum tank and thentransferred to the 4000 liter bioreactor. Typically, the bacteria aremaintained in each inoculum tank for a period of one to three daysbefore transfer to the next inoculum tank or the bioreactor 10. Atstart-up of the bioreactor 10, a culture of the bacteria must betransferred to the bioreactor 10 and allowed to grow before thecontinuous production of ferric ions can begin. Typically, this periodis one to three days.

It should be noted that in addition to growth in liquid medium, growthof the bacteria can also be carried out on solidified medium asdescribed in Yates and Homes, Two Families of Repeated DNA Sequences inThiobacillus ferrooxidans, Journal of Bacteriology, 169:1861-1870(1987).

The invention not only enables ferric ions to be produced fromacidophilic, chemoautotrophic bacteria but also enables the bacteria tobe continuously maintained in the bioreactor 10 to provide a low costcontinual source of the ferric ions. To be so maintained, the bacteriarequire a source of metabolic energy which is supplied in the form of afeed solution. The feed solution is preferably a ferrous sulfate growthmedium containing ferrous sulfate in an aqueous solution maintained at apH of about 1.5-2.0 with sulfuric acid. The ferrous iron concentrationof the feed solution has an effect on the growth rate of the organisms.The ferrous iron concentration of the feed solution varies dependingupon the requirements of the subsequent use of the ferric ions.Typically, ferrous ion concentration is within the range of 1 g/l to 15g/l.

The feed solution is stored in a feed tank 24 and supplied to theinterior chamber 20 of the bioreactor 10 from the top of the tank 12 viaa pump 28 and an influent conduit 30. Preferably, the feed tank 24 isconstructed of a polypropylene material, the pump 28 is constructed witha corrosion resistant magnetic drive, and the conduit 30 is constructedof a polyvinyl chloride material. A control valve 32 is positioned alongthe conduit 30.

The tank 12 includes an overflow port 34 located near the top portion 36of the side wall 16. The port 34 maintains a constant solution level inthe tank 12 by allowing any solution overflow to exit the tank 12 viathe port 34.

The bioreactor 10 also includes a recirculation conduit 38 having firstand second ends 40 and 42 respectively. The first end 40 is incommunication with the interior chamber 20 of the tank 12 through a port44 in a lower portion 46 of the side wall 16. The second end 42 of therecirculation conduit 38 is in communication with the interior chamber20 through a port 48 in the top portion 36 of the side wall 16. A pump50 is connected to the recirculation conduit 38 to perform therecirculation of the contents in the bioreactor 10 in the direction ofthe arrow (FIG. 1). The port 48 also enables the tank 12 to be drainedfor cleaning, maintenance, etc.

Addition of the feed solution to the bioreactor 10 is controlled bymonitoring the bacterial population cell density. Cell density is aparameter that can be monitored and controlled to optimize theproduction of ferric ions. Preferably, cell density is maintained in therange of approximately 5×10⁸ cells/ml (OD₅₀₀ =0.100) to 1×10⁹ cells/ml(OD₅₀₀ =0.200).

To monitor cell density, a Lasentec™ particle size analyzer (PSA) 52 isemployed. The PSA 52 is preferably placed along the recirculationconduit 38 and is operatively connected to the control valve 32. Whenthe PSA 52 determines the bacterial cell density in the bioreactor 10 istoo high, the PSA 52 sends a signal to the control valve 32 and thecontrol valve 32 opens allowing feed solution to enter the bioreactor 10via the conduit 30. When the PSA 52 determines cell density is too low,the PSA 52 sends a signal to the control valve 32 and the control valve32 closes thus shutting off the supply of feed solution to thebioreactor 10. When the control valve 32 is closed, the pump 50recirculates the contents in the tank 12 through the conduit 38.Alternately, the PSA 52 can be placed in the interior chamber 20 of thetank 12 to monitor cell density.

To maintain a continuous growth of the bacterial population in thebioreactor 10, control of pH is required. Preferably, the pH ismaintained in the range of 1.5-2.0, and more preferably, in the range of1.7 to 1.9. pH control is also necessary to keep the iron in a solubleform. To maintain pH control, the tank 12 preferably has three ports 54in the side wall 16 capable of supporting pH probes 56. One probe 56a islocated in port 54a in the lower portion 46 of the side wall 16, oneprobe 56b is located in port 54b in a middle portion 58 of the side wall16 and one probe 56c is located in port 54c in the top portion 36 of theside wall 16. The pH probes 56 are preferably specially designed forfermentors such as the pH probes manufactured by Omega of Stamford,Connecticut as model number Omega® PHE-6820.

The pH probes 56 are in communication with a controller 60 such as asuitably programmed microprocessor. When the pH in the interior chamber20 of the tank 12 is above a set range, such as the range 1.5-2.0, thecontroller 60 activates a control valve 62 that is in communication withan acid supply tank 64 containing an acid such as 1ON sulfuric acid. Thechoice of acid may depend upon the requirements of the subsequentapplication of the generated ferric ion solution. The acid enters theinterior chamber 20 of the tank 12 via a conduit 66 to lower the pH ofthe solution in the tank 12. When the pH is lowered enough to fallwithin the set range, the controller 60 sends a signal to close thecontrol valve 62.

If the pH level in the tank 12 is too acidic and outside the set range,the controller 60 activates a control valve 68 that is in communicationwith a base supply tank 70. The choice of base may depend upon therequirements of the subsequent application of the generated ferric ionsolution For example, ammonium hydroxide can be used. The base entersthe interior chamber 20 of the tank 12 via a conduits 72 and 66 to raisethe pH of the solution in the tank 12. When the pH is raised to fallwithin the set range, the controller 60 sends a signal to close thecontrol valve 68.

To facilitate growth of the bacterial population, tank agitation isperformed. The agitation is accomplished through recirculation of tanksolution from the bottom of the interior chamber 20 to the top throughthe recirculation conduit 38 as discussed above. Tank agitation is alsoaccomplished by sparging, for example, a 7.5% carbon dioxide/92.5% airmixture through a sparging tube 74. optionally, a mechanical mixer 76such as an impeller rotating at 200-800 rpm can be utilized to addfurther agitation. The dissolved oxygen of the solution in thebioreactor 10 is monitored using a probe 78 such as a dissolved oxygenprobe manufactured by Phoenix Electrode Company of Houston, Tex. Thesparging tube 74 supplies the bacteria with a source of oxygen andcarbon dioxide. The carbon dioxide can be generated by acidification ofcalcium carbonate and introduced into the tank 12 at preferably a 7.5%by volume mixture with the compressed air.

Temperature in the bioreactor 10 can also be controlled if necessary.The temperature can be conventionally maintained by circulation of waterthrough a heating and cooling jacket (not shown) which surrounds thetank 12. Preferably, the temperature of the solution in the tank 12 ismaintained in the range of 25°-40° C.

After the start-up population of Thiobacillus ferrooxidans is placed inthe interior chamber 20 of the bioreactor 10, the bacteria are allowedto grow without the addition of feed solution until an increase in thecell density is detected by the PSA 52. Thereafter, the feed solution isadded to the bioreactor 10 and the system parameters are measured andmonitored to keep them in their respective optimal ranges.

The ferric ion solution produced in the bioreactor 10 exits the tank 12via an effluent line 82. A small amount of the bacteria also exit thebioreactor 10 with the ferric ion solution. The bacteria exiting thebioreactor 10 do not affect the continuous maintenance of the bacteriathat remain in the bioreactor 10.

To monitor and control the relative amount of ferric ions produced, anoxidation-reduction potential (ORP) probe 84 is used such as a digitalpH/millivolt meter 611 available from Orion Research of Boston, Mass.The ORP probe 84 is positioned along the recirculation conduit 38.Preferably, the ORP value of the solution in the bioreactor 10 ismaintained at less than 520mV to prevent the formation of precipitate,and most preferably, is maintained at less than 450mV.

All conduits used are autoclavable and are Masterflex silicone tubing.Preferably, the conduits 66 and 72 are Masterflex 6409-14 tubing and theconduits 30 and 38 are Masterflex 6411-16 tubing.

Monitoring of some or all of the bioreactor parameters can beaccomplished using a microprocessor controller such as that manufacturedby Omega as model number Omega® PHCN-37. Alternatively, some or all ofthe bioreactor parameters can be monitored and maintained with AdvancedFermentation Software (AFS) available from New Brunswick Scientific Co.,Inc. of Edison, N.J.

As shown in FIG. 1, the ferric ion solution produced exits thebioreactor 10 via the effluent line 82 and can thereafter be used in anapplication 86 such as that requiring the treatment of metal or sulfurbearing material. With the use of a 2400 liter tank, approximately 4800liters of ferric ion solution can be generated every 24 hours.

In some applications, the interaction of the ferric ion solution withthe material to be treated results in the regeneration of ferrous ionsthat can be recycled to the bioreactor 10 to serve as a continual orclosed loop source of energy for the growth of the organisms in thebioreactor 10. Material toxic to the bacteria such as chloride ionsneeds to be removed from the solution before introduction into thebioreactor 10. Further, the concentration of ferrous ions needs to beadjusted to eliminate the effect which varying concentrations of ferrousions could have on the growth of the bacteria in the bioreactor 10. Afilter 88 is also employed to ensure that no particulates enter thebioreactor 10.

Specifically, when the converted ferrous solution is to be recycled backthrough the bioreactor 10, the organisms can be and preferably areslowly adapted to the recycled solution which contains increased levelsof toxic contaminants. A gradual and systematic exposure of the bacteriain the bioreactor 10 to the components of the recycled solution is adesirable acclimatization procedure. For example, with the gradualintroduction of the recycled ferrous solution, it is possible to achieveapproximately 90% replacement of the original feed solution with therecycled ferrous ion solution, with a resulting reduced bacterial growthrate of approximately fifty percent. Accordingly, by slowly adapting theorganisms to the recycled ferrous solution, growth of the organism doescontinue in the environment of increased toxins. The ability toacclimate the organisms to the recycled solution further decreases thecost of production of the ferric ion solution making the invention evenmore cost effective.

With respect to the leaching applications for the ferric ion solution,it is the belief that direct attachment of the bacteria to the materialto be treated is not necessary and is not the main mechanism by whichsuch bacteria are able to oxidize material. The role of thechemoautotrophic bacteria are to produce the ferric ions that can besubsequently used in leaching. Accordingly, the ferric ions generated bythe bioreactor 10 create a constant supply of leachant for a widevariety of applications.

Referring now to FIG. 4, a typical integrated system for producing andutilizing ferric ions is shown. The bioreactor 10 containing bacteriacontinuously produces ferric ion solution that is then used in anoxidation application 90. It should be noted that several bioreactors 10can be used in parallel to supply the necessary amount of ferric ionsolution for a particular application.

In general, the ferric ion effluent produced by the bioreactor 10 can beused in the two types of leaching processes: percolation leaching andagitation leaching. In percolation leaching, the ores, concentrates orscrap metals are stacked on an impermeable pad or rubbilized in place.The ferric ion solution is then applied to the solids by gravitypercolation or pumped through the material. The leach liquor iscollected at the bottom of the heap. In agitation leaching, the ferricion solution is mixed with finely ground solids to form a slurry. Thesolids and liquid phases are then separated using any conventionalsolid/liquid separation technique.

Both leaching applications generate two products; a treated solid 92(leached material) and a liquid which is rich in metal (preg solution).The solids may be disposed (as in the case of recovering primary metalsfrom a solid), set aside for further processing (as in the pretreatmentof precious metal ores) or sold (as in the case of cleaning coal). Thesolution may be stripped of the metal value using cementation on scrapiron or steel wool, chemical precipitation as a sulfide or hydroxideprecipitate, electrowinning to plate a specific metal or treated byother methods specific to recovering non-ferrous metals in solutiondenoted as 94 in FIG. 4. The stripped solution will contain primarilyferrous ions. Optionally, an on-line atomic absorption orspectrophotometry 96 can be used to automate the dilution or addition offerrous ion to maintain a steady iron concentration in the system and asolution clarifier 98 can also be employed. As described above, the pregsolution can be returned to the bioreactor 10 for regeneration of ferricion solution to continue the cycle.

Three examples of the types of oxidation applications that utilizeferric ions are set forth below. Many additional applications are notlisted and the examples set forth hereinafter are not intended to belimiting to the potential uses of the bioreactor 10 and the ferric ionsproduced therein.

A. Oxidation of primary metal from an ore or scrap. The ferric ionsolution produced by the bioreactor 10 is of a sufficient strength todirectly leach the metallic, sulfidic and oxidic forms of copper, zinc,lead, iron, mercury, nickel, arsenic, cadmium, cobalt, antimony,bismuth, silver, tin, etc. from metal scrap or naturally occurringminerals. These metals are soluble in ferric ion solution and may berecovered from the solution using any of several conventionaltechniques.

B. Pretreatment step to enhance recovery of precious metals. Theminerals described above may be associated with precious metal ores.These minerals are detrimental to the recovery of precious metals andmust be oxidized or removed prior to conventional processing. The ferricions produced by the bioreactor 10 can be used in an oxidativepretreatment process of this purpose.

As exploration increased in the past few decades, many low grade golddeposits have been discovered which contain significant amounts ofsulfide minerals. These minerals hinder the recovery of gold by directcyanidation and must be removed in a pretreatment oxidation process. Theferric ion effluent produced by the bioreactor 10 can be used todirectly oxidize these minerals prior to cyanidation as follows. Thegold-bearing ore containing sulfidic minerals is crushed and placed onan impermeable pad. The ferric ions generated in the bioreactor 10 orseveral bioreactors are applied to the crushed ore to oxidize thesulfide minerals. After the oxidation is complete in several days, theheap is rinsed with a high pH solution followed by cyanide solutionswhich dissolves the precious metal. Such a process can increase goldrecovery from 5-7% in untreated ores to as high as 50% in treated ores.If the preg solution in the oxidation step is to be recycled to thebioreactor(s) 10, the solution should be treated to minimize oreliminate the toxic components prior to entry into the bioreactor 10.Secondary metal recovery could also be realized by stripping the metalsfrom the solution prior to recycle to the bioreactor 10.

The result is an enhanced recovery of gold from ores previouslyconsidered too refractory to treat at a low cost. The process isapplicable in the pretreatment of refractory carbonaceous ores as well.

C. Removal of gangue or hazardous compounds to increase the value ofproducts. Eastern United States coal is characterized as containinghigher concentrations of sulfur due to large amounts of pyrite whichcontribute to the coal's high sulfur content. By removing the pyritethrough oxidation with ferric ions, the coal is more useful to largecoal consumers, such as power plants, metal-producers, etc. In essence,the use of ferric ions eliminates the production of sulfur dioxidethrough aqueous processing of the coal.

Conversely, western United States coal typically contains higher amountsof mercury-bearing minerals. Ferric ions can treat the elemental mercuryand/or mercuric sulfide, either of which may be present in this coal.

By continuously growing chemoautotrophic bacteria in a controlledenvironment such as the bioreactor 10, several advantages are gained.Some of the more important advantages include, but are not limited to,the following. First, the ferric ion effluent from the bioreactor 10will contain some bacteria which are flushed with the solution. Thesebacteria may continue to thrive within the material being leached,possibly enhancing the oxidation process. However, even if these cellsdo not survive, the oxidation process will continue based on thecontinuous source of ferric ions from the bioreactor 10.

Second, the generation of ferric ions can be optimized through thecareful control of the conditions in the bioreactor 10. Third, thehealth and viability of the bacteria in the bioreactor 10 is maintainedthrough monitoring of system parameters. Fourth, automation of thebioreactor 10 keeps the cost of operation and production of ferric ionsminimal. Fifth, the preg solution can be treated and returned to thebioreactor to assure a steady supply of ferrous ions to maintain thebacteria in the bioreactor 10.

We claim:
 1. A method for continuously maintaining a population ofThiobacillus ferrooxidans bacteria to generate ferric ions comprisingthe steps:a) inoculating a vessel with a population of Thiobacillusferrooxidans bacteria; b) supplying a feed solution to said vesselwherein the feed solution contains growth media and an aqueous solutionof ferrous sulfate; c) supplying carbon dioxide to said vessel; d)monitoring the pH of the contents of the vessel with a pH probe andmaintaining the pH of the contents of the vessel in the range of 1.5 to2.0 with the addition of acid or base; e) monitoring the cell density ofthe bacteria with a particle size analyzer and maintaining the celldensity in a predetermined range by controlling the amount of feedsolution supplied to said vessel; f) agitating the contents of saidvessel; and g) recirculating said contents of said vessel.
 2. The methodfor continuously maintaining a population of Thiobacillus ferrooxidansas set forth in claim 1, wherein said step of supplying carbon dioxideto said vessel includes the use of a sparging tube.
 3. A method forcontinuously generating ferric ions comprising the steps:a) establishinga population of acidophilic, chemoautotrophic bacteria which oxidizeferrous ions into ferric ions in a vessel; b) supplying a feed solutionto said vessel wherein the feed solution contains growth media and anaqueous solution of ferrous sulfate; c) monitoring the pH of thecontents of the vessel with a pH probe and maintaining the pH of thecontents of the vessel in the range of 1.5 to 2.0 with the addition ofacid or base so that said bacteria generate ferric ions; d) withdrawingsaid generated ferric ions from said vessel; e) monitoring the celldensity of the bacteria with a particle size analyzer and maintainingthe cell density in a predetermined range whereby additional ferric ionscan be generated by controlling the amount of feed solution supplied tosaid vessel; and f) recirculating said contents of said vessel.
 4. Theprocess for continuously generating ferric ions as set forth in claim 3wherein said acidophilic, chemoautotrophic bacteria include at least oneof Leptospirillum ferrooxidans, Sulfolobus acidocaldarius, Sulfobacillusthermsulfidooxidans and Thiobacillus ferrooxidans.
 5. The process forcontinuously generating ferric ions as set forth in claim 3 wherein saidacidophilic, chemoautotrophic bacteria include Thiobacillusferrooxidans.
 6. A method for oxidizing metal or ore comprising thesteps:a) supplying a vessel housing a population of acidophilic,chemoautotrophic bacteria which oxidize ferrous ions into ferric ions;b) supplying a feed solution to said vessel wherein the feed solutioncontains growth media and an aqueous solution of ferrous sulfate; c)monitoring the pH of the contents of the vessel with a pH probe andmaintaining the pH of the contents of the vessel in the range of 1.5 to2.0 with the addition of acid or base so that said bacteria generateferric ions; d) monitoring the cell density of the bacteria with aparticle size analyzer and maintaining the cell density in apredetermined range whereby additional ferric ions can be generated bycontrolling the amount of feed solution supplied to said vessel; e)withdrawing said generated ferric ions from said vessel; and f)oxidizing the metal or ore with said ferric ions, wherein supplementalferrous ions are produced from the oxidation of the metal or ore; and g)recirculating said supplemental ferrous ions produced from the oxidationof the metal or ore into the feed solution.
 7. The method for oxidizingmetal or ore as set forth in claim 6 wherein said bacteria areThiobacillus ferrooxidans.
 8. The method for oxidizing metal or ore asset forth in claim 6 wherein said acidophilic, chemoautotrophic bacteriainclude at least one of Leptospirillum ferrooxidans, Sulfolobusacidocaldarius, Sulfobacillus thermosulfidooxidans and Thiobacillusferrooxidans.
 9. A method for solubilizing metal from ore, alloys orscrap comprising the steps:a) supplying a vessel housing a population ofThiobacillus ferrooxidans bacteria; b) supplying a feed solutioncontaining ferrous ions to said vessel so that said bacteria generateferric ions, wherein the feed solution contains growth media and anaqueous solution of ferrous sulfate; c) monitoring the pH of thecontents of the vessel with a pH probe and maintaining the pH of thecontents of the vessel in the range of 1.5 to 2.0 with the addition ofacid or base so that said bacteria generate ferric ions; d) monitoringthe cell density of the bacteria with a particle size analyzer andmaintaining the cell density in a predetermined range by controlling theamount of feed solution supplied to said vessel; e) withdrawing saidgenerated ferric ions from said vessel; and f) solubilizing the metalfrom ore, alloys or scrap with said ferric ions, wherein supplementalferrous ions are produced during the solubilization of the metal fromore, alloys or scrap; and g) recirculating said supplemental ferrousions produced during the solubilization into the feed solution.